Description

A route whose path ends in a parameter and has children is enough for AngularNodeAppEngine to return a cross-origin redirect from the request path alone — no application redirectTo, no guard, no resolver, no X-Forwarded-* control, no authentication:

GET /.;/(//evil.test)   ->   302 Location: //evil.test

A browser resolves //evil.test as protocol-relative and leaves the application's origin.

The Router serializes the navigation to /.//evil.test. That string starts with /., not //, and resolves to the application's own origin, so neither guard in ServerPlatformLocation.replaceState() fires , but WHATWG normalization pops the preceding segment, leaving pathname === '//evil.test'.
@angular/ssr then emits that pathname as the redirect target without normalizing it:

const { pathname, search, hash } = envInjector.get(PlatformLocation);

if (urlToRenderString !== finalUrl) {
  redirectTo = [pathname, search, hash].join("");
}

A pathname may legally begin with //, so nothing upstream is malformed. The comparison arm normalizes its inputs; the redirect arm does not.

Minimal Reproduction

import { Component } from "@angular/core";
import { RouterOutlet, Routes } from "@angular/router";

@Component({ imports: [RouterOutlet], template: "<router-outlet />" })
export class TenantLayout {}

@Component({ template: "tenant page" })
export class TenantPage {}

export const routes: Routes = [
  {
    path: ":tenant",
    component: TenantLayout,
    children: [{ path: "**", component: TenantPage }],
  },
];

npm run build
NG_ALLOWED_HOSTS=localhost PORT=4000 node dist/repro/server/server.mjs

for p in '/.;/(//evil.test)' '/xx;/(//evil.test)' '/acme'; do
  printf '%-22s ' "$p"
  curl -s -o /dev/null -w '%{http_code}  %{redirect_url}\n' --path-as-is "http://localhost:4000$p"
done

/.;/(//evil.test)      302  http://evil.test/
/xx;/(//evil.test)     302  http://localhost:4000/xx//evil.test
/acme                  200

The second line is the control: . replaced with xx removes the dot-segment pop and the redirect stays on the origin. /.;/(/evil.test) gives location: /evil.test, also same-origin , both the popping dot segment and the leading empty segment are required.

Minimal Reproduction

See https://github.com/SkyZeroZx/angular-ssr-router-open-redirect

Your Environment

22.2.0

Anything else relevant?

The canonical application-side mitigation does not stop it. A returnUrl check requiring a relative, non-protocol-relative path accepts /.;/(//evil.test), so an application that validated correctly still redirects off-origin:

export const returnUrlGuard = (route: ActivatedRouteSnapshot) => {
  const target = route.queryParamMap.get("returnUrl") ?? "/";
  if (!target.startsWith("/") || target.startsWith("//")) {
    return true;
  }
  return inject(Router).parseUrl(target);
};

With { path: "login", component: Login, canActivate: [returnUrlGuard] } added to the config above:

/login?returnUrl=%2F.%3B%2F(%2F%2Fevil.test)   302  location: //evil.test   <- accepted
/login?returnUrl=https%3A%2F%2Fevil.test       200  no redirect             <- rejected
/login?returnUrl=%2F%2Fevil.test               200  no redirect             <- rejected

router.navigateByUrl(target) behaves the same. This moves the payload into a query parameter, so the delivered link is /login?returnUrl=... rather than a visibly odd path. search is concatenated unchanged, so query parameters reach the redirect target.

Also reachable without a literal . or // in the request: GET /%2e;/(/\evil.test) returns location: //evil.test, since %2e is decoded and \ normalized after any inspection of the raw path.

Affected shapes: a parameter route with children, plain or lazy, whose descendants can match a two-segment group, at URL depth ≤ 2. Childless parameter routes, children without a **, a ** child using redirectTo.